Accurate color rendering of original images is an important criterion for digital imaging devices, such as, digital cameras, scanners, photocopiers, facsimile machines, and multipurpose machines. Inaccurate color rendering is known to cause colors that are different in the original images to appear the same in the rendered images and to also cause colors that are similar in the original images to appear different in the rendered images. Therefore, rendering digitally captured images such that they accurately match the colors in the original images is critical to providing visually accurate digital reproductions of the original images.
The color accuracy of a digital imaging device depends on a number of hardware and software components contained in the digital imaging device. Some of the more critical components include filters, image sensors, illuminants, and color correction computations. The color correction computations typically include the use of look-up tables, non-linear functions or linear matrix techniques, to modify the raw color signals, for instance, RGB signals, of the capture images to create a more accurate reproduction of the capture images.
The components of conventional digital imaging devices are selected to maximize color accuracy without regard to the noise in the digital imaging devices. This approach is typically suitable for digital imaging devices having only three channels, where the three channels are not highly correlated, and where the image sensor has a good-signal-to noise ratio. However, if any of these conditions is not met, maximizing the color accuracy will cause the noise in the digital imaging devices to be greatly amplified.
The noise is amplified because light is formed of photons that arrive at the sensors at discrete intervals of time instead of continuously, and light therefore fundamentally includes noise. Measurement of the light leads to a noise source referred to as shot noise. Shot noise at each color sensor is independent and uncorrelated to all the other color sensors. Therefore, when the raw color signal from a sensor is computed with the color correction, the shot noise is amplified by coefficients contained in the computation. Larger coefficients generally lead to larger noise amplification and result when either the imaging device has more than three-channels or the three-channels are highly spectrally correlated, and the coefficients are designed to maximize accuracy. Therefore, maximizing color accuracy also has the undesirable side effect of maximizing color noise.
An improved approach to designing and constructing digital imaging devices that do not suffer from the drawbacks associated with conventional digital imaging devices discussed above would therefore be beneficial, particularly for imaging devices configured to process more than three channels of information.